On producing color prints, a variety of colors and tones is normally obtained by successively applying inks of Cyan(C), Magenta(M) and Yellow(Y) (primary colors in subtractive mixture) as well as Black-(B.sub.K) ink, which mainly compensates dark part density graduation at desired proportions. The amount of each ink to be applied is directly controlled by a corresponding color separation plate which, therefore, defines the final color tone of the printed matter. Prior to final printing, test prints or proofs are made to check the final color tone expected to result from use of the color separation plates, the plates being modified or reproduced if necessary. This procedure is known as color proofing.
A conventional color proofing process consists of making printing plates from each of a plurality of color separation films Y, M, C, B.sub.K, test printing with the printing plates and corresponding printing inks to obtain a proof and, if deemed necessary either from the proof or from the conditions of the original color film and/or each color separation film Y, M, C, B.sub.K, modifying the separated color films (hand retouching) or, after adjusting tone and/or color separation conditions, reproducing new color plates and producing a new proof. This means that after the color plates are modified or reproduced, each printing plate corresponding to the separated color films must be made and a test printing with the printing plates must be done with a proof-printing press. Therefore, the conventional proofing method is a trial and error process and involves expenditures of much time and cost.
To overcome these difficulties, several electronic color proofing systems have been realized. One such system records each video signal obtained from each separated color film Y, M, C, B.sub.K through a TV camera, retrieves the recorded video signals synchronously, performs necessary calculations with the signals in an electronic circuit for generating color tones identical to those of a print to be produced, and displays the result of the calculations on a color monitoring screen. Another system employs flying-spot scanners which scan a set of separated color films synchronously to display a color image without recording any video signal. Another new system, known as a layout scanner system, records on a memory device such as a magnetic disc each color density signal obtained from color components by a color scanner. The system transfers each color density signal to a refresh memory to display color tone simulating that of a print to be produced.
However, it should be noted that there exists a recognizable gap between the spectral characteristics of ideal inks and those of actual printing inks which contain unwanted color absorbing factors which are usually expressed as ink impurities. Therefore, when displaying a color image on a color monitoring screen by using video signals obtained from each film of separated color through a TV camera, a circuit is required to calculate how much each monochromatic color printed with a relevant color plate will be affected by the impurity of the corresponding ink. Additionally, a circuit is also required to calculate how discolored a final multi-color print will be, printed with all the color plates and corresponding inks.
In this connection, several methods of performing these color calculations are already disclosed.
One of the methods is described as follows. That is, at first, preliminarily digitizing density signals of separated colors corresponding to printing inks, normally Cyan(C), Magenta(M), Yellow(Y) and Black (B.sub.K), on a necessary density graduation for expressing desired color tone, secondly preparing a look-up-table memory loaded with values which, verified by actual test printing and subsequent comparison, correspond to all the possible combinations of every density graduation level of each color separation film. Thereafter, Red(R), Green(G) and Blue(B) signals which will embody color tone identical to that of a print to be produced can be directly retrieved from the look-up-table memory, by using address signals utilizing the separated color density signals.
In the above method, if an equation is available which is based on fundamental characteristic values and is capable of predicting the color tone to be produced by actual printing, the equation may be applied instead of said digitized signals.
On the other hand, when digitizing each color density signal, maintaining of natural color tone normally requires a density graduation of about 256 levels for each of the C, M, Y and B.sub.K color plates. This means that the number of combinations of every density level of each color plate is (256).sup.4 .apprxeq.4.times.10.sup.9, which requires too large a memory capacity. For this reason, as disclosed in Japanese Patent Application No. 56-93013, U.S. Ser. No. 378,792, by the same applicant, preparation of a conveniently thinned out look-up-table memory along with the interpolation (in the above case, fourth-degree-calculations) is proposed.
With this improved method, the actual equation to be used for interpolation is rather simplified. However, an application of the equation to hardware still involves some problems such as complexity of multiplying circuits and requirements of comparison circuits for adapting to various situations. All of these problems will be multiplied, especially when more than four printing inks are used, resulting in complex and expensive hardware.
Another method of performing the aforesaid calculations replaces the look-up-table memory with real time calculation which predicts the color tone to be printed from each color density signal.
To embody the real time calculation, several methods have been previously suggested. One of them utilizes Neugebauer's equation or its derivative equation, as disclosed in Japanese Patent laid open No. 49-12910, U.S. Ser. No. 235,296). This method includes the steps of determining quantities of inks to be used from each of the separated color signals Y, M, C, B.sub.K, calculating area rates for every possible combination of inks, multiplying the area rates by each specific reflex factor and summing these values to predict the mean color tone to be printed with a plurality of inks.
With this method, the more printing inks are used, the more terms are needed in the calculations to obtain the area rates. This inevitably involves complicated settings of factor data for the terms in the equations and consequently requires a complex and expensive system.
Though the Neugebauer's equation is comparatively clear, it is also well known that, when the equation is applied for prediction of color tone to be printed with a typical halftone dot density of about 150 lines per inch, the equation gives little success.
Modification and correction of the Neugebauer's equation in an attempt to obtain successful application would lead to introduction of exponential terms in equations, as seen in said Japanese Patent laid open No. 49-12910, resulting in still more complicated calculations.
There are other methods for real time calculation, as disclosed in Japanese Patent Publication Nos. 54-38921, 54-38922 and 56-26015. The method of said Japanese Patent Publication Nos. 54-38921 or 54-38922 include the steps of determining the quantities of inks to be used from each density signal of separated colors and summing the values of density signal of R, G, B of each printing ink selectively to obtain the total value of each separated color component density wherein it is assumed that the values of density signal of R, G, B of an ink increases proportionally with the amount of the ink. However, in the printing process there is a characteristic known as additivity-law failure, owing to a characteristic whereby a portion of a print with multiple inks is seen too thick of its color factor on a CRT monitor provided that a higher performance of realization of color scanning for a portion of a printed matter with one ink is achieved. Therefore, practically, an approach of under-color-removal as used in color scanners is introduced into the above method to rectify the signals of color components R, G, B of the portion printed with multiple colors.
The method of the said Japanese Patent Publication No. 56-26015 includes the steps of selectively summing the values of density signals of R, G, B, calculated from the amount of each ink to be used to obtain the total value of each color component density, calculating correcting terms against additivity-law failure by multiplication and adding the obtained values to the total value of each composed color density to predict the color tone of a print to be produced.
All these aforesaid methods for real time calculation, however, require circuits for detecting portions with multiple separated colors on a print and circuits for multiplications of each density value of color components. In addition, an exact prediction of the color tone to be printed requires an appropriate selection of factors for every term used in the equations and, when more than four printing inks are used, the increased number of combinations of separated colors would render these methods impracticable.
In particular, with the method using correcting terms against additivity-law failure, those correcting terms in equations which have been found relatively small through taking the amount of impurity of ink into account are neglected while this omission could be better avoided for correctness.
It is, accordingly, an object of the present invention to overcome the disadvantages in the prior methods by providing a method and system that will utilize simpler calculations leading to simpler calculating circuits and realize the display of a better color image of a color printed matter on a screen.
The calculations used in the present invention rely upon the so-called density-saturation theory. By applying the theory, each R, G, and B color signal to be input to a color picture tube can be obtained through selective summing of Value L=-log (1-Di/k) for each value of color component density as shown in equations (3), (4) and (5) below, wherein k is a constant of density-saturation point; Di is each value of color component density signal contained in a color density signal corresponding to the amount of a printing ink, such as C, M, Y or B.sub.K ; and L is a common term used in the equations (3), (4) and (5) which are derived from the density-saturation theory.
More specifically, the system of the present invention will have a memory loaded with the Value L=-log(1-Di/k) corresponding to one of the color components corresponding to a printing ink to be used and the values will be retrieved from the memory to be summed for each total density of color components, which will finally control a color picture tube.